Due to environmental concerns and newly enacted rules and regulations, petroleum products are expected to meet lower and lower limits on contaminants, such as sulfur and nitrogen. New regulations require the removal of sulfur compositions from liquid hydrocarbons, such as those used in gasoline, diesel fuel, and other transportation fuels. For example, US EPA Tier 3 gasoline regulations will permit up to about 10 ppm sulfur in gasoline.
Hydrodesulfurization is a hydrotreating process often used for removal of sulfur from olefinic naphtha streams by converting sulfur in the feed to hydrogen sulfide via contact with suitable catalysts. The value of naphtha is dependent upon its octane value. Octane is increased by the presence of mono-alkenes. However, di-alkenes and alkynes present a processing problem in a hydrodesulfurization reactor because they easily polymerize and gum up equipment and transport lines and deactivate hydrodesulfurization catalyst. High temperature processing of olefinic naphtha, however, may result in a lower grade fuel due to saturation of mono-alkenes leading to an octane loss.
A selective hydrogenation reactor is typically provided upstream of a hydrodesulfurization reactor to remove di-alkenes and alkynes while minimizing mono-alkenes saturation. Better processes and apparatuses are needed for selective hydrogenation of di-alkenes and alkynes.